The present invention relates generally to fuel control systems, and more particularly, to a method of determining and adjusting the concentration of ethanol content of the fuel in a flexible fueled vehicle.
Environmental and energy independence concerns have stimulated the development of alternative transportation fuels, such as alcohol fuels, for use in automobiles. Alcohol fuels include methanol and ethanol. A flexible fueled vehicle capable of operating on gasoline, or alcohol fuel, or any mixture of the two fuels, is desired. Modifications to the air/fuel mixture are necessary when switching operation of the vehicle from one fuel value to another. Air/fuel ratio in an internal combustion engine design is typically considered to be the ratio of mass flow rate of air to mass flow rate of fuel inducted by an internal combustion engine to achieve conversion of the fuel into completely oxidized products. The chemically correct ratio corresponding to complete oxidation of the products is called stoichiometric. If the air/fuel ratio is less than stoichiometric, an engine is said to be operating rich, because too much fuel is being burned in proportion to the amount of air required to achieve perfect combustion. Conversely, if the air/fuel ratio is greater than stoichiometric, an engine is said to be operating lean because too much air is being burned in proportion to the amount of fuel required to achieve perfect combustion. Alcohol fuels have a lower air/fuel ratio than gasoline of stoichiometric; accordingly, the engine must be compensated for in the rich direction as a percentage of alcohol in the fuel increases.
In the absence of a fuel composition sensor, the alcohol concentration is determined using the exhaust gas oxygen feedback system. Under some cold starting conditions, the oxygen sensor, however, is incapable of being used for air/fuel ratio corrections. Oxygen sensors take a predetermined amount of time to warm up to a reliable state, particularly on cold starts. However, under some cold starting conditions, learning the ethanol concentration is desired.
The engine roughness, as measured by the second derivative of the engine speed, can be used to indicate lean fuel operation, and therefore can be used to determine the extent of which the air/fuel mixture needs to be enriched to protect driveability. However, variations in engine roughness caused by manifold air pressure (MAP) and engine speed (RPM) changes must also be considered to accurately determine the cause of engine roughness, otherwise a lean operation may be mistaken for true engine misfire, and also true engine misfire could be mistaken for a lean operating condition. Either of these conditions could negatively affect driveability. Therefore, it would be desirable to provide a method for determining the percent alcohol content of fuel by measuring engine roughness associated with MAP and RPM changes, and adding fuel to minimize engine roughness during the transition from one fuel mixture to another.